The invention concerns a method for regulating radio frequency (RF) signals in a nuclear magnetic resonance (NMR) system, comprising a spectrometer, a control loop, and an NMR probe head with RF components, wherein the spectrometer comprises a transmitter which transmits RF signals at measuring frequencies and with a transmission power. The NMR probe head comprises at least one RF oscillating circuit which is tuned, by means of RF components, to the resonance frequency of a type of nucleus to be investigated, wherein the RF oscillating circuit comprises at least one RF coil which is disposed around a measuring sample and, by transmitting RF signals, is used to excite nuclear spins in a sample and to receive NMR signals resulting from this excitation, wherein the NMR probe head may contain further RF components which may be components of the RF oscillating circuit, of the coupling and the filter networks.
In NMR spectrometer systems, radio frequency pulses are irradiated onto a measuring sample by means of a transmitting coil or a transmitting/receiving coil, and the time-resolved response from the measuring sample is detected. The magnetic field component of the RF fields (“magnetic field”) thereby couples with the spin system. In many cases, several measuring frequencies are also connected to one single transmitting/receiving coil by means of a tuning network. In addition to the use of coils having one or several windings, which may be designed in the form of saddle coils or solenoid coils, the use of resonator structures such as Birdcage or Alderman-Grant resonators is also possible. Other types of resonators, such as e.g. coaxial or transmission-line resonators, are used less frequently. The term “coil” below means any form of transmitting or transmitting/receiving structure which is used to generate an RF field in a measuring sample.
With a defined magnetic field, i.e. a defined amplitude of the current in the coil and defined transmission frequency f0, the excitation pulses have a characteristic duration (pulse duration p1) in order to rotate the nuclear spin magnetization of a certain type of nucleus with a resonance frequency fS=f0 through 90°. The pulse durations associated with further pulse angles can generally be easily calculated from this 90° pulse duration. When the transmission frequency f0 of the RF pulses differs from the resonance frequency fS of the spins, the pulse angle will not be 90° for a given current amplitude. The excitation width of an RF pulse is generally inversely proportional to its pulse duration.
A current pulse must be generated in the coil to generate the radio frequency magnetic field in the measuring sample. The coil is generally connected to form an oscillating circuit using capacitors and/or a tuning network, and is tuned to the resonance frequency of the nuclear spins. This oscillating circuit is connected to a transmitter by means of a coupling network via transmission lines with defined impedance (normally 50Ω). The coupling network is used to adjust the impedance of the oscillating circuit to the impedance of the transmission line. With optimum adjustment, a wave is transmitted without reflections. In case of mismatch, part of the wave is reflected at the coupling network. Such reflections occur at all impedance breaks. For this reason, it is important to adjust the oscillating circuits to the different measuring frequencies of one measuring head and adjust the impedances to the transmission lines. With perfect adjustment in the stationary limiting case, the overall transmitted power is transferred to the oscillating circuit and dissipated there.
The impedance of an oscillating circuit at resonance is inversely proportional to its Q-value (with fixed inductance of the coil). However, when lossy measuring samples are used, the Q-value of an NMR detection system decreases due to the additional losses in the measuring sample. For this reason, the impedance transformation must be adjusted. This adjustment is called “matching”. Furthermore, the resonance frequency of an oscillating circuit changes due to the electric field components which penetrate through the measuring sample. It must be readjusted to the transmission frequency after change of a measuring sample in order to prevent reflections and ensure optimum reception sensitivity. This process is called “tuning”.
The current in the oscillating circuit will decrease by increasing the loss resistance/reducing the Q-value upon insertion of a lossy measuring sample into the measuring head for a given transferred power. The generated radio frequency magnetic field has a smaller amplitude and in order to obtain the same flip angle of the nuclear spins, either the duration of the pulses must be extended or the transmission power must be increased.
In NMR spectrometers according to prior art, the pulse duration is adjusted in order to compensate for the additional losses. When lossy measuring samples are used, this extension of the excitation pulses reduces the excitation bandwidth compared to measuring samples without loss.
The reason for the extension of the pulse durations is that, in general, the transmitted power is limited to protect the measuring head from destruction e.g. by dielectric breakdowns. The measuring head therefore realizes the shortest pulse duration pmin for a 90° pulse only with loss-free measuring samples, but with lossy measuring samples the pulse durations can be considerably longer, p1>pmin. FIG. 10 shows the extension of the pulse duration (p1/pmin) as a function of the salt concentration in the sample for a typical NMR measuring head.
The extension of the pulse duration linearly reduces the spectral width during excitation. The longer the pulses, the poorer the usual spectroscopic quality. Spectral regions which are far away from the excitation frequency, are not or are only insufficiently excited such that the signal intensities at that location are reduced or reception of a signal is no longer possible. It is definitely normal that the pulse duration may be doubled or even tripled over the relevant range of solvent losses. This, however, also means that only half or even only one third of the spectral width is available for excitation. This is particularly and extremely critical for inversion pulses, since these already have a significantly reduced excitation width compared to a 90° pulse. Lossy measuring samples frequently fail to achieve sufficient excitation with inversion pulses such that so-called adiabatic pulses must be used instead. These, however, require a considerably higher dissipated overall power and have a much longer overall duration during which the spin system evolves, and they cannot be used in any situation. For these reasons, it is often desirable to do without such pulses.
One further problem consists in that power amplifiers are generally not adjusted to 50Ω but have a lower resistance in order to increase their efficiency. When the measuring head and certain cable lengths between amplifier and transmitting coil are mismatched, the effectively delivered power may be higher compared to adjustment to 50Ω. This increases the voltage/current in the coil or the network, although the nominally transmitted power is identical. For this reason, the transmission power must be limited to a value that offers sufficient reserves to also cover any mismatches.
For short transmission pulses with high resonator Q-values, the adjustment to 50Ω is not the ideal power adjustment. Due to the high resonator Q-value, transient oscillations occur during which the ideal power adjustment greatly deviates from that of the stationary state. With high (Q>1000) or very high (Q>10000) Q-values and short pulses (p1<10 μs), the stationary state is not obtained during the pulse duration such that the shortest pulse angles cannot be obtained with 50Ω matching.
In practice, there are also situations in which it is not possible to calculate from a known pulse with flip angle α1, pulse duration p1 and power P1, a pulse with a flip angle α2, pulse duration p2 and power P2 without errors. The reasons therefor are e.g. non-linearities of the coil materials that are used over the power range P1 to P2, which, in addition to increased dissipation, also cause mismatches and thereby increased reflection. Such non-linearities may occur e.g. through heating or with superconducting coil materials which are operated close to their critical currents. With very high Q-values or high transmission power, a situation may even arise in which with constant transmission power, a pulse with large flip angle cannot be correctly calculated from a pulse with small flip angle. In the first case, this is due to reflection of the transmitted signal on the resonator due to mismatch during the transient oscillations as mentioned in the previous paragraph. In the second case, the heating due to dissipation of the high power during the pulse is responsible.
It is therefore the object of the invention to propose a method, an NMR probe head, and an NMR system which compensate for occurring losses, in particular without reducing the pulse duration, which correct errors in the calculation of pulses or which automate determination of pulses.